In the related art, various vehicle driving systems are known (e.g., refer to PTL 1 to 3). Among these, in PTL 1, two engines of a first engine and a second engine are provided as a power source, only the first engine is operated when a required torque is small and the output is input to a transmission, the second engine is added and operated when the required torque is great, and the output of both engines is combined and input to the transmission, and thereby, the required torque is produced according to an optimal condition corresponding to the state of a load, and fuel consumption of a vehicle is reduced.
In PTL 2, power of an engine which includes two pistons of differing stroke (the engine may be considered as substantially two engines) is input to a transmission in parallel via a one-way clutch, and the power is transmitted to an output shaft.